Spectrum of the /3 Rays excited by 7 Rays. 



285 



J3 rays. It was obvious 011 inspection of the plates that the 

 stronger groups of excited /3 rays corresponded very nearly 

 in position with the primary /3-ray spectrum of radium B. 

 The values of Up for the stronger groups of /3 rays excited 

 in lead are given in the following table ; for the purpose of 

 comparison, we have added a list of some of the primary 

 ,j3 rays of: radium B, taken from the paper of Rutherford and 

 Robinson. It will be seen that within the limits of experi- 

 mental error, the velocities of the rays are the same in 

 each case. 



Table 



of /3 rays 



excited in Lead. 





Up of excited (3 ray. 



Intensity. 



lip of primary j3 ray. 



Intensity. 



3610 



f. 







3250 



f. 







2990 



s. 







2735 



m.f. 







2225 



in. 



2235 



s. 



2130 



f. 



2140 



f. 



2000 



f. 



1990 



m.s. 



1935 



v.s. 



1925 



v.s. 



1825 



m.f. 



1815 



m. 



1750 



m.f. 



1752 



m.s. 



1670 



v.s. 



1G60 



v.s. 



1560 



f. 







1400 



s. 



1392 



v.s. 



1240 



in. 







1150 



m.f. 







1080 



m. 







1010 



m.s. 







950 



s. 



950-914 (group) 



in. 



870 



v.s. 







800 



s. 



798 



m.s. 



[v.s. = very strong ; s. = strong; m.s.— moderately strong ; in.= moderate 

 intensity; m.f. = medium faint ; f.= faint. 



Dependence of intensity and velocity of the excited /5 rays 

 upon the material. 



It is of great interest to determine whether the velocity 

 of the excited /3 rays depends on the material through which 

 the 7 rays pass. For this purpose, comparative measure- 

 ments were made with screens of aluminium, silver, gold, 

 and lead surrounding the emanation tube. With the 

 screen of aluminium 0*6 mm. thick, it was impossible to 

 detect any groups of /3 rays against the general fog of the 

 plates : with silver the lines were faint and difficult to 

 measure, but the lines from lead and gold were well marked 



